UNIT 2

MICRO AND NANOFABICATION

TECHNIQUES

 Bottom Up Methods
 Top Down Methods
 Syllabus
Nanotechnology, Bottom up and top down
methods of synthesis- Self-assembly-
lithography techniques, etching - Ion
implantation, surface micromachining-
LIGA process-CVD technique
 Bottom Up Methods of Synthesis
• The Bottom-Up Approach involves building complex structures
starting at the atomic or molecular level.
• Atoms or molecules naturally arrange themselves into desired
structures using chemical or physical processes.
• Examples:
 Self-assembly: Atoms/molecules arrange themselves into functional
structures without external direction.
 Chemical Vapor Deposition (CVD): Gaseous reactants form solid
material on a substrate.
 Top Down Methods of Synthesis
• The Top-Down Approach involves starting with a bulk material and
removing parts of it to create micro- or nano-scale structures.
“Start big, remove to get small.”
• This is opposite to the bottom-up approach which builds structures
atom by atom or molecule by molecule.
• Examples:
Lithography
Etching
Ion Implantation
 Self-Assembly Technique
Self-assembly is a process where molecules, atoms, or
nanostructures spontaneously organize into ordered and
functional structures without external guidance, driven
by natural interactions like hydrogen bonding, van der
Waals forces, electrostatic forces, or hydrophobic effects.
"Nature’s way of building structures from the bottom
up."
Type Description Example
Once formed, the structure
Static Self-Assembly DNA origami, micelles
remains unchanged.
Structures form,
Protein folding, cellular
Dynamic Self-Assembly disassemble, or reassemble
membrane formation
based on stimuli.

Steps in a Self-Assembly Process

Key Driving Forces
 Preparation of building blocks (molecules,
 Hydrogen bonding
 Van der Waals interactions nanoparticles).
 Controlled environment (pH, temperature).
 Electrostatic interactions
 Spontaneous organization into desired
 π-π stacking (aromatic ring stacking)
 Hydrophobic/hydrophilic structure.
 Stabilization (e.g., cross-linking or solid
interactions
support).
Self-Assembly technique Video Links
• https://
youtu.be/1gco4bhsBPk?feature=shared
• https://youtu.be/1kWApU3o6ko?feature=s
hared
 Lithography Techniques
• Used to transfer patterns onto a substrate.

• Photolithography:
▫ Uses UV light to create patterns on a photoresist layer.

▫ Common in IC and MEMS fabrication.

• Electron Beam Lithography (EBL):

▫ Uses focused electron beams for high-resolution patterning.

▫ Slower but enables <10 nm features.

▫ Used in nanotechnology labs and prototyping.

Photolithography
• Photolithography is a crucial process in the production of
semiconductors that is used to create unpredictable models on
silicon wafers.
• The communication begins with the utilization of a
photoresist, a photosensitive material, to the external layer
of the wafer.
• After that, ultraviolet light is passed through a mask that
contains the desired pattern and is applied to the resist-coated
wafer.
What is Photoresist in Photolithography?
▫ Photoresist is an essential component of
photolithography.
▫ It goes through compound changes upon openness to
light, taking into consideration the particular
evacuation of specific regions during improvement.
▫ There are two principal types:
 Positive Photoresist
 Negative Photoresist
 Positive Photoresist and Negative Photoresist

Feature Positive Photoresist Negative Photoresist

UV light increases UV light decreases

Exposure Reaction
solubility solubility
Exposed areas are Unexposed areas are
Developer Action
dissolved dissolved
Pattern matches the mask Pattern is the inverse of
Final Pattern
opening the mask opening
Resolution Typically higher resolution Typically lower resolution

Fine feature definition, IC Thicker resist layers,

Common Usage
fabrication MEMS, packaging
Positive Photoresist
• Mechanism: Upon openness to bright (UV) light, positive photoresist
particles go through a substance change, turning out to be more solvent in
an engineer arrangement.
• Advantages: Offers high goal and astounding edge definition. Its aversion
to UV light takes into consideration fine example creation.
Negative Photoresist
• Mechanism: Negative photoresist turns out to be less solvent in an
engineer upon openness to UV light.
• Advantages: It is ideal for intricate designs. It resists chemical attack and
displays clear difference among uncovered and unexposed areas.
Light Sources
• Photoresists are sensitive to light with wavelengths ranging
from 300 to 500 nm.
• The most popular light source for photolithography is the
mercury vapor lamp (310 to 440 nm).
• Deep UV light has wavelengths of 150 to 300 nm, while the
normal UV light has wavelengths between 350 and 500 nm.
• In special applications for extremely high resolution, x-rays
are used 4 to 50 Å (1 angstrom Å = 0.1 nm or 10⁻⁴ µm).
Photolithography Process
Steps in Photolithography

Step Description
The wafer is thoroughly cleaned to remove particles, organic residues,
1. Substrate Cleaning
and contaminants to ensure proper adhesion of the photoresist.
A thin, uniform layer of photoresist is spin-coated onto the wafer
2. Photoresist Coating
surface.
The wafer is gently heated (typically ~90–100°C) to remove solvents
3. Soft Bake
and solidify the photoresist, enhancing adhesion and uniformity.
A photomask with the desired circuit pattern is precisely aligned over
4. Mask Alignment
the wafer.
UV light (typically 310–440 nm from a mercury vapor lamp) is
5. Exposure projected through the mask, altering the solubility of the photoresist in
exposed areas.
 A second baking step (~110–120°C) stabilizes the chemical
6. Post-Exposure Bake changes in the photoresist, enhancing image quality and reducing
standing wave effects.
A developer solution (e.g., KOH or TMAH) is used to dissolve
7. Development either the exposed or unexposed regions (depending on positive or
negative photoresist).
The wafer undergoes high-temperature baking (~120°C) to harden
8. Hard Bake the remaining photoresist and improve its resistance during
etching.
The revealed parts of the substrate are either etched (material
9. Etching/Implantation removal) or ion-implanted (material modification) to create the
actual pattern.
The residual photoresist is removed using plasma descumming
10. Photoresist Stripping or chemical solvents, leaving behind the patterned substrate for
further processing.
Advantages of Photolithography
 High-Quality Patterns
 Efficient Production Applications of Photolithography

 Cost-Effective  Semi-conductor Manufacturing

 Versatile  Electronics
 Accurate Alignment  Circuit Boards

 Fast Processing  Optics

 Medical Devices
Disadvantages of Photolithography
 Nanotechnology
 Size Limits
 Complex Process
 Expensive Equipment
 Costly Pattern Design
 Environmental Concerns
 Etching
• Etching removes material from desired areas using physical or
chemical methods.
• Purpose:
▫ To create permanent patterns on a substrate (after
photolithography).
▫ To shape micro components in MEMS and microsystems.
• Examples:
▫ Making cavities in silicon dies for micro pressure sensors.
▫ Forming silicon membranes and diaphragms in micro valves.
• Types of Etching:
▫ Dry Etching (Plasma Etching): Physical method.
▫ Wet Etching: Chemical method.
1. Dry Etching
• It is the removal of materials from a substrate using gaseous
chemicals (no liquid/chemical rinsing involved).
• Common techniques:
▫ Plasma etching
▫ Ion milling
▫ Reactive Ion Etching (RIE)
• Plasma Etching
▫ Plasma = ionized gas with free electrons and positive ions.
▫ Plasma is usually generated using an RF (Radio Frequency)
source.
▫ A reactive gas like CCl₂F₂ is introduced into the plasma.
▫ The plasma: Has reactive neutrals formed when the gas ionizes.
▫ Contains ions that bombard and remove material.
• Working
Plasma contains:
 Ions (+ve)
 Reactive neutrals (uncharged)
Ions hit the substrate surface and the sidewalls.
Reactive neutrals hit mostly sidewalls.
The etching is mainly due to chemical reaction (by
reactive neutrals) and physical bombardment (by
ions).
Etching is faster vertically than sideways → creates
anisotropic etching (directional).
• Etching Speed:
▫ Conventional dry etching: Slow (~0.1 μm/min or 100
Å/min).
▫ Plasma etching: Fast (~2000 Å/min).
▫ Speed improves because:
 Gas molecules travel straight paths (long mean free path).
 High-energy reactions due to plasma.
• Plasma Etchants for Different Materials:
Material Conventional Chemicals New Chemicals
Silicon & SiO₂ CCl₂F₂, CF₄, CF₃F, CCl₂F₂,
C₂F₆
Silicon Nitride (Si₃N₄) CCl₂F₂, CHF₃ CF₄/O₂, CH₃CHF₂
Polysilicon Cl₂ or BCl₃/CCl₄ HBr, SF₆, Br₂
Gallium Arsenide (GaAs) CCl₂F₂ SiCl₄/SF₆, CF₄
• Deep Reactive Ion Etching (DRIE)
▫ DRIE is an advanced dry etching process used in MEMS fabrication.
▫ It allows for deep, narrow trenches with almost vertical sidewalls (θ ≈
0°).
▫ Used where high aspect ratio (A/P = Depth/Width) is needed - up to
30:1 or more.
How DRIE Works
• Plasma ions and reactive neutrals are used.
• Protective layers (like polymers or silicon dioxide) are deposited
on sidewalls.
• These layers prevent lateral etching, allowing for deep and straight
cuts.
• Plasma and protection are alternated to maintain shape and depth.
 Feature Regular Plasma Etching DRIE
Sidewall angle (θ) Wide (slanted walls) Close to 0° (vertical walls)
Aspect ratio (A/P) Less than 15 Up to 200
Sidewall protection Not used Uses protective coatings
Etch rate Faster (~2000 Å/min) Slower (2–3 µm/min)
Application Low-depth trenches Deep, narrow trenches
Materials Used for Sidewall Protection:

Material Selectivity Ratio Achievable Aspect Ratio (A/P)

Polymer — 30:1

Photoresist 50:1 —

Silicon dioxide 120:1 200:1

• Selectivity ratio: How well the etching distinguishes

between material and mask/protection.
• Aspect ratio (A/P):
• Gases Used in DRIE:
▫ Common gas: Fluoropolymers (nC₄F₈) in Argon
plasma.
▫ These gases help form polymer layers on sidewalls.
▫ High-density plasma is used to alternate between
etching and protecting.
• Performance of DRIE:
▫ High aspect ratio of A/P = 30 to 200 achieved.
▫ Very straight sidewalls (θ = ±2°).
▫ Etch depth up to 300–380 µm.
▫ Etch rate = 2–3 µm/min.
2. Wet Etching
• Wet etching is a material removal process that uses
liquid chemical solutions to dissolve away specific
parts of a material (usually metals, oxides, or
semiconductors) from a substrate.
• Principle
• Wet etching works based on chemical
reactions between the material and an
etchant (chemical solution).
• The chemical selectively reacts with the
material to be removed, converting it into
a soluble compound that is then washed
away.